It is known that during completion and workover operations within the area of subsea operation, weak links are utilized.
The function of the weak link is to provide a given and controlled method of ultimate separation of the riser, if all other known methods have failed, and the operator is in a worst case mode. Such a mode may arise due to extreme weather, failure of anchoring or positioning system, black-out (power failure) on the vessel, or by other unforeseen means.
In such a worst case scenario it is vital that the vessel is able to passively disconnect from the wellhead and infrastructure on the sea bed, in order to remove the vessel from the conduit to the reservoir and to avoid uncontrolled breaking of the riser and subsequent possible blow-out. This is also required to ensure safety to the personnel onboard. Such a passive disconnection is achieved by use of a weak link.
The failure of the weak link (i.e. the disconnection caused by a weak link in its normal mode of operation) can be attributed to several prime failure modes, and in these can be further related to the operational window and physical position of the vessel.
A heave compensation system for a riser system compensates for variations in the vessel's vertical position in relation to the seabed and inherent upward pull, provided by the vessel. It ensures that buckling/tear-off of the riser system is avoided. If this heave compensation system fails, a failure mode known as ‘compensator lock up’ takes place. This then results in application of tension or compression on the riser system, due to changing vertical position of the vessel, caused by wave motion.
Such failure causes buckling or over-pull and unless the over-pull is limited by a weak link then the operator runs the risk of damage to the subsea systems, including the wellhead, and ultimately risks substantial environmental pollution due to leakage of hydrocarbons, and in worst case a blow-out. The weak-link thus has to fail in a mode whereby the vessel is “on station” (in the correct position) but has a compensator that has locked up, resulting in the vessel applying its own heave (vertical motion due to wave patterns) directly to the riser. Ideally a weak link should protect for such a case.
If the system used to maintain on-station position, either via anchoring or dynamical positioning using thrusters, should fail then a situation known as drift-off or drive-off will occur. This results in the vessel rapidly leaving the green (safe) operation window and entering the yellow (unsafe) and red (danger) zones. These are determined by actual vessel position, relative to a nominal purely vertical riser system.
In case of drift-off, the weak link should be able to fail ultimately, with a permanent break and separation of upper and lower riser sections. The most important aspect is to completely and immediately passively disconnect the vessel from the subsea infrastructure, and hence avoid any damage to the wellhead and/or vessel and personnel.
Conventional weak links are constructed most often by the use of two flanged sections of riser that are bolted together at the flanges using tension bolts, whereby the bolts are designed to fail at a given load.
The riser itself may be in a depressurized state (atmospheric pressure) during the course of operation, or it may be filled with oil and/or gas at pressure. Due to the end-cap effect of the riser system, the pressure present in the riser will exert a tension force in the riser equal to the pressure multiplied by the cross sectional area of the pressurized medium. This tension force acts at every cross section of the riser, hence also acts at the tension bolts. Due to varying pressure (from atmospheric during initial installation) and through to full bore pressure, the tension bolts will be subjected to varying pre-tensions in the riser.
This results in the weak link being susceptible to failure at varying mechanical tensions (T fail=T bolts−T end cap). Given the constant value of the bolt tension failure load, and the variation of pressure, the operator will be depending on a weak link with varying and uncontrollable tension limits. This in practice reduces and affects the safe mode of operation.
Hence the tension load (end cap) due to variations of bore pressure has to be balanced; so called “pressure balance” whilst not compromising the normal operation of disconnection/opening of the weak link.
US patent publication number 2011/0127041A1 attempts to teach such pressure balance by providing a riser weak link having an upper housing and a lower housing which are releasably attached by studs. The studs are designed to break at predefined load. There is also a pressure application device which provides a coupling force on the upper housing to counter balance the separation force applied by well pressure. This ensures that the only separation force acting on the top portion of a riser system attached to the upper housing, is the tension applied by the surface vessel.
However, the prior art acknowledged in the preceding paragraph, has a major draw back. On release of the studs at predefined tension load, the upper housing and lower housing are likely to separate with a sudden snap or jerk. Such recoiling of upper housing and lower housing and the corresponding riser portions attached to each, leave potentialities of damage to sub-sea infrastructure and equipment and to personnel on the surface, wide open.
Apart from the disadvantage in the preceding paragraph, the prior art does not teach specifically and explicitly the adaptability of the weak link to effectively function when the riser system is in operation in subsea condition (i.e. weak link operating in weak mode) and also when the riser system is lowered and retrieved; i.e. weak link operating in strong mode when the gripping force between the two principal releasably connected components of the weak link, need to be strengthened.
Accordingly, there is a long felt need for a weak link for riser systems which has a pressure balancing mechanism which can effectively work with a damping mechanism, so that the upper riser portion and the lower riser portion on disconnection by release of the connection tool such as studs or release bolts, are separated in a controlled manner, limiting substantially any sort of recoiling.
There is also a need for a weak link for riser systems which has a simple mechanism for effectively functioning under varying conditions, when the riser system is in operation in subsea condition and also when the riser system is lowered and retrieved. It is common to use a riser as a lowering means for a valve tree (XMT), by attaching the XMT below the emergency disconnect package (EDP) & lower riser package (LRP) at the lower end of the riser. This is a very heavy assembly, and the inclusion of a conventional weak link poses potentially disastrous overloading risks, particularly in poor weather. Alternatively, the operational window is very narrow.
The present invention meets the above mentioned needs and other associated needs by providing a weak link for riser systems having a pressure balancing mechanism which can effectively function in association with a damping mechanism for controlled and smooth separation of the two main releasably joined components of the weak link, each having portions of risers, connected at lower end and top end respectively. The weak link according to the present invention can also effectively function in both weak mode and strong mode as explained before, in a very simple manner.